The present embodiments relate to diagnostics for programmable logic controllers (PLCs).
PLCs control automated processes, such as controlling operations in a factory. A PLC connects with one or more loads, such as motors, actuators, and/or sensors. The connections allow control of the loads by the PLC. However, the connection and/or the load may fail. For example, a wire becomes disconnected or shorted to ground or a high voltage. To deal with possible connection problems, self-contained diagnostic circuitry is included in the PLC.
False diagnostics are to be avoided, as a false diagnostic may unnecessarily shut down a process. However, inductive loads may cause false diagnostics. When a switching output is connected to an inductive load, there is an energy transfer as the output switches to the off state. This energy transfer disrupts the diagnostic circuitry until the energy has been dissipated. This inductive commutation may cause false diagnostic events to be detected.
To avoid false diagnostic events, the diagnostic detection is performed only after a delay the energy to fully dissipate. The delay is based on a maximum possible inductance to be connected with the output. This wait occurs whether an inductive load is present or not and whether the inductive load has a maximum inductance or not. As a result, delay is introduced in the control and/or diagnostic process.